This invention relates to luminous dot sighting instruments and more particularly to such instruments used in connection with a telescope on a firearm as an aid to sighting the firearm on a target.
Optical sighting instruments having a sighting tube have included cross hairs in the tube to define the optical center of the instrument. The user, such as a firearm marksman, looks into the sight and places the cross hairs on the target in the view to aim the firearm. The cross hairs are illuminated by light from the view. At twilight, in haze or fog and at other times when visibility is low, the cross hairs are difficult to see, and at night they cannot be seen at all. One solution to this problem has been the luminous dot sighting instrument.
A luminous dot sighting instrument is used to view a target and is usually attached to a rifle or other firearm to sight the rifle on the target. The luminous dot is generated inside the instrument and is superimposed on the view of the target by a transparent mirror in the instrument. The mirror is transparent to light from the target scene and reflects light from the luminous dot. In this way the luminous dot is superimposed on the target scene as viewed through the instrument and serves the purpose of the cross hairs. Thus, this type of sighting instrument is called a reflex luminous dot sighting instrument. It does not depend on light from the view and can be made as bright as desired by the user by controlling the brightness of the luminous dot source.
The instrument is often a sighting tube containing the transparent mirror (window/mirror) and the luminous dot source is a light emitting diode (LED) inside the tube. With the sighting tube are also a battery, brightness control circuit and switch for the LED, usually fixedly attached to the tube. Or the sighting instrument may be a telescope, in which case the LED is inside the telescope and the battery, control circuit and switch are carried on the outside of the telescope as part thereof. In either case, the instrument is attached to a rifle to aid the user in pointing the rifle to hit the target viewed through the instrument.
In such reflex luminous dot sighting instruments, with or without a telescope, the LED is contained in the instrument wherein light is projected from the LED onto a tilted window or lens that has a mirror coating so that light from the view passing through the lens is Joined by light from the diode that reflects from the mirror coating. The user looking into the instrument attached to his rifle sees the target view with the luminous dot at the center and points the rifle to place the dot on the target in the view and fires the rifle. With a properly mounted and adjusted sighting instrument the user can quickly view the target area, put the dot on the target and fire the rifle with great accuracy.
Heretofore, a number of reflex type luminous dot sighting instruments, some with telescopes and some without, have been used with adjustments for range and windage to be made by the firearm user. The adjustments for range and windage are adjustments in elevation angle and azimuth angle, respectively, of the instrument with respect to the firearm and are set by the user using his estimates of range and windage. In all of these, the target view and the luminous dot are combined by the tilted window or lens with a mirror coating and light from the target view passes through the tilted lens while light from the LED that forms the luminous dot reflects from the lens mirror coating. Also, in all of these, the LED is contained within the sighting tube or telescope and is powered by a battery in circuit with the LED and a brightness control and/or switch, all attached to and carried by the tube or telescope. Thus, each sighting tube or telescope comes equipped with the LED, battery and brightness control and/or switch.
Where a telescope is included, the light from the LED is focused by the lens/mirror on the same image plane as the target view so that the user sees the target view and the luminous dot all in focus at the same plane with the luminous dot precisely at the center of the target view. The LED is enclosed within the telescope by an aperture that directs the LED light as a narrow angular cone to the center of the tilted lens. Thus, the luminous dot appears to the user the same size, shape and color for all target views, at the center of the target view, whether the target is far or near (long or short range elevation) and whether there is any adjustment for windage, left or right.
Such a luminous dot sighting instrument including a telescope is described in my U.S. Pat. No. 5,205,044, issued Apr. 27, 1993, entitled Luminous Dot Sighting Instrument.
In a reflex luminous dot sighting instrument, as mentioned above, the brightness of the dot is independent of ambient light and can be increased by increasing electric power to the LED. However, as the dot is made brighter to the observer it loses definition and becomes larger. The applicant has discovered that this loss of definition is caused mainly by undesired reflections of the LED light before it reaches the tilted window or lens. The undesired reflections appear to increase as power to the LED is increased. This is probably due to increased divergence of the luminous dot beam as power is increased.
In the past, the loss of definition with greater dot brightness has been partly eliminated by using high quality windows and lenses coated with high quality mirror reflecting layers that selectively reflect the dot source light and not the target scene light and the use of black dull coating inside the sighting tube and inside the telescope.